(1) Field of the Invention
The present invention relates to a rotor blade, to a silent rotor, to an aircraft provided with such a rotor, and more particularly to a tail rotor for a helicopter. The invention also relates to a method of obtaining such a rotor blade.
The invention is thus situated in the narrow technical field of aircraft rotors.
(2) Description of Related Art
For example, a helicopter generally has a single main rotor driven mechanically by at least one engine, the main rotor providing the helicopter with lift and propulsion.
This type of helicopter is also provided with a tail rotor that performs an anti-torque function in order to compensate the torque created by the rotation of the main rotor, the tail rotor exerting a transverse thrust.
In addition, the tail rotor enables the pilot to control yaw and turning movements of the helicopter by exerting positive or negative transverse thrust.
A non-ducted tail rotor is known for performing this function and is referred to for convenience as a “conventional tail rotor”. Conventionally, the non-ducted tail rotor is mounted on one side of the top end of a tail fin or at an end of a tail boom of the helicopter.
Such a non-ducted tail rotor is in widespread use. Nevertheless, it is also possible to use a ducted tail rotor, e.g. as known under the trademark Fenestron®.
A ducted tail rotor is then arranged in a duct provided in the bottom portion of the vertical tail fin of a helicopter, with the longitudinal axis of the duct being substantially perpendicular to the vertical plane of symmetry of the helicopter.
As a result, the streamlined structure of the vertical tail fin of the helicopter surrounds said duct and thus surrounds the tail rotor, which explains why such a tail rotor is said to be ducted. It should be observed that the streamlined structure is referred to by the person skilled in the art as a “fairing”.
The streamlined structure then protects the ducted tail rotor against impacts created by elements external to the aircraft. It also increases safety for ground personnel by preventing such personnel being injured by the tail rotor in its duct.
The streamlined structure surrounding the duct in which the ducted tail rotor is arranged prevents noise from being transmitted forwards, downwards, and towards the rear of the rotorcraft, whereas a non-ducted tail rotor transmits noise in all directions.
Conventionally, a ducted rotor has a fairing that defines a duct. A rotor having a hub carrying a plurality of blades then rotates within the duct.
The hub is driven in rotation by a transmission gearbox fastened to the fairing by support bars that are arranged downstream from the rotor. These support bars may be shaped so as to constitute stationary vanes for guiding the stream of air downstream from the rotor. Under such circumstances, the assembly comprising these support bars is sometimes referred to an air stream “guide-stator”.
The terms “upstream” and “downstream” are defined relative to the flow direction of air through the duct.
It can be understood that the term “ducted rotor” is used below to mean the assembly comprising in particular the fairing, the rotor as such, the gearbox, and the vanes located downstream from the blades.
The ducted rotor thus includes in particular the fairing and the elements present within the duct that is defined by the fairing.
Ducted rotors present advantageous functional characteristics.
Nevertheless, although the noise emitted by a ducted rotor is minimized, in particular in a forward direction, such a ducted rotor is still liable to generate noise.
The wake created by each blade of the ducted rotor can generate noise on impacting the vanes of the guide-stator.
In the state of the art, document FR 2 719 549 describing a ducted rotor is known. That ducted rotor has blades that are angularly distributed with irregular azimuth modulation determined by a sinusoidal relationship.
In addition, the vanes of the guide-stator are inclined so that they do not extend radially in order to minimize interaction of a blade with each vane.
Document FR 2 719 552 describes vanes for a guide-stator. According to that document, the tabs for fastening the root and/or the tip of a vane may be inclined or bent relative to a plane that extends in the span and chord directions of each vane.
Document FR 823 433 describes an aircraft having a non-ducted propeller and plano-convex type wings.
Document U.S. Pat. No. 7,874,513 describes a vertical takeoff and landing aircraft.
That aircraft has ducted rotors, each provided with a plurality of blades. The blades have leading and trailing edges that are C-shaped.
Document US 2007/201982 describes a ventilator and therefore does not lie in the technical field of the invention.
Furthermore, the wake generated by a rotor blade is liable to generate potentially troublesome noise, both in the context of a ducted rotor and also in the context of a non-ducted rotor.
For example, the Applicant has observed that the wake from a blade of a main rotor for providing a rotorcraft with lift can, surprisingly, impact against another blade of the same main rotor, in particular while the rotorcraft is moving in translation both forwards and downwards.
Independently of the nature of the rotor, the wake generated by a blade is therefore liable to give rise to sound annoyance, at least in certain situations.
The technological background also includes the following documents: FR 1 134 736; EP 2 336 022; U.S. Pat. No. 1,949,611; and GB 191500049 A, A.D. 1915 (Freeman Nat Harris).
Document FR 1 134 736 presents a blade having an undulating side edge.